1. Field of the Invention
This invention relates to an addition curing silicone rubber composition which cures into a rubber sheet having surface adhesive property, and a pressure-sensitive adhesive rubber sheet obtained therefrom.
2. Background Art
Pressure-sensitive adhesives (PSA) are characterized in that they readily adhere to surfaces of objectives under as small pressures as finger pressing. These adhesives are utilized as SPA products including PSA tapes (e.g., Scotch® tape and Cello® tape), electrically insulating vinyl tapes, masking tapes and PSA sheets. For PSA tapes and sheets, PSA compositions based on natural rubber or synthetic rubber are often used. Undesirably, these PSA compositions are subject to thermal or optical degradation and tend to lose adhesion at low temperatures.
In contrast, silicone pressure-sensitive adhesives have excellent heat resistance, freeze resistance and electrical properties inherent to silicone and maintain adhesion without impairing these properties. They are thus widely used as PSA products where a high level of reliability is required. Typical silicone PSA compositions include a silicone PSA composition comprising a condensation product of a polysiloxane having (CH3)3SiO0.5 units and SiO2 units with a dimethylsilicone gum and an addition type silicone PSA composition comprising an alkenyl-containing organopolysiloxane and an organohydrogenpolysiloxane, which cures through hydrosilylation reaction. See JP-B 54-37907 corresponding to U.S. Pat. No. 3,983,298 and JP-A 63-22886 corresponding to U.S. Pat. No. 4,774,297.
Since these PSA compositions are typically coated to substrates to form thin films whose surface adhesion is utilized, no reference is made to the rubber hardness and strength. If these compositions are actually formed into rubber sheets, the sheets have sufficient adhesive property, but lack rubber strength. Then the sheets are not suited as jig-fixing sheets or shock-absorbing sheets. JP-A 8-134427 describes to specify the content of alkenyl groups in a resin. Low contents of alkenyl groups fail to provide sufficient rubber strength whereas high contents of alkenyl groups lead to insufficient adhesion. It is also known to add inorganic fillers such as powdered silica to silicone to enhance its rubber strength. Since the addition of such fillers, even in minor amounts, detracts from transparency, the composition is no longer used in the optical application, for example, as a shock-alleviating material in an image display apparatus.